Existing soil displacement tools are normally provided to carry out relatively small diameter tunneling or piling operations. They are pneumatically driven and comprise an elongate member incorporating a cylinder arranged with its axis coincident with the longitudinal axis of the tool, a piston being caused to reciprocate and, in normal operation, strike an anvil plate at the forward end of the tool to cause the tool to move through the soil in which it is placed as a result of the impact imparted to the tool by the piston.
Such tools are wasteful in energy in that the compressed air which is fed thereto is exhausted to atmosphere from the rear of the tool. Additionally, they are relatively low on power.
They suffer from the further disadvantage that they are on many occasions lost as a result of being trapped down the hole they have produced as a result for example, of collapse of the hole behind the tool. This problem is mitigated in certain tools by providing a reversing action so that the piston is caused to impact the rear end of the tool to cause it to move in a reverse direction but this has called for relatively complicated and expensive valve gear and has not been particularly efficient.
A further disadvantage which often occurs in a piling operation is that as the tool is withdrawn from the hole it has produced the hole collapses prior to the introduction of a cementitious grout which is to be fed into the hole to produce the pile.